The physics of a lassoed satellite Atlantis mission tied to test of technology

July 31, 1992|By Douglas Birch | Douglas Birch,Staff Writer

Astronauts on the space shuttle Atlantis plan to go fly a kite Monday, lofting a half-ton satellite on a tether resembling a white bootlace 12 miles long.

This is no diversion for a summer afternoon. It's a test of technology that future shuttle astronauts may use to drag instruments through the Earth's rarefied upper atmosphere, to loft satellites into orbits thousands of miles high, to generate power and to create enormous space-based antennas.

But controlling lassoed satellites in near-zero gravity is no simple task. The publication Satellite News, quoting an unnamed NASA engineer, has reported that flying and retrieving the 5-foot-diameter, beach ball-shaped satellite "may prove to be one of the more challenging and dangerous tasks ever undertaken by a shuttle crew."

So the Atlantis astronauts, scheduled for launch today, are prepared to cut the tether if, say, the satellite threatens to turn into an orbiting wrecking ball and collide with the shuttle, or if the tether sticks while being reeled in, preventing the shuttle bay doors from closing.

In principle, the physics of tethered satellites are simple and well understood. In practice, the movements of two orbiting objects strung together like a bola can be exasperatingly complex.

"It becomes a very large equation when you try to put all the motions that are possible into it," said the project's program manager, Thomas D. Stuart of the National Aeronautics and Space Administration.

But he added that the risk is not as great as sometimes depicted.

The crew has practiced delicately maneuvering the satellite, which is equipped with an array of tiny thrusters, and the shuttle to keep the tether taut during the 33-hour experiment.

The satellite is expected to drift up and behind the shuttle, which will orbit the Earth at an altitude of about 200 miles.

Mr. Stuart said the tether will be reeled out and in so slowly -- never faster than about seven feet per second and much slower closer to the orbiter -- that the crew will have plenty of time to react to anything unexpected.

NASA engineers expect the tether to exhibit something called the "skiprope effect": a tendency to swing in a circle, like a jump rope, between the shuttle and the satellite.

It might also make what NASA is calling a "yo-yo" motion, in which where the tether would go slack and taut in quick succession, causing the orbiter and satellite to bounce at the ends of the tether.

If the astronauts don't have their hands full stabilizing the cable, they are expected to provoke some mild vibrations so they can study how to control them.

The $191 million satellite, developed jointly by NASA and the Italian space agency ASI, will gather data on the Earth's ionosphere, an area where thinly distributed atoms of air from the Earth are electrically charged by high-energy particles from the sun. And it will map changes in the Earth's magnetic field as it sweeps around the planet.

The $91 million tether is designed to be electrically conductive: As it flies through the Earth's magnetic field, it is expected to generate 5,000 volts of electricity.

On future space flights, a similar tether could be used to provide power to the spacecraft. (The shuttle now uses fuel cells to provide electric power.)

The crew of the Atlantis includes Baltimore native Marsha S. Ivins, a NASA astronaut who also flew on a January 1990 shuttle mission.

Ms. Ivins, 41, will serve as the mission's photographer, and she plans to use still, video and IMAX motion picture cameras to document everything from the results of a growth hormone experiment to the deployment and operation of the tethered satellite.

The satellite-on-a-rope concept is an ancient one, by aerospace standards. In 1895, Russian inventor and rocket expert Konstantin Tsiolkovsky envisioned cables attached to 22,000-mile-high towers that could pull passengers into space.

Space scientists like to point out that, in theory anyway, orbiters could drop "skyhooks" attached to long cables down to the Earth's surface and hoist objects back into space.

NASA has long been interested in the idea. In 1966, during separate missions, the Gemini 11 and Gemini 12 spacecraft were tethered to upper stages of their Agena rockets by 100-foot Dacron straps. During both flights, the straps jitterbugged erratically.

The late Giuseppe Colombo of NASA's Goddard Institute in New York proposed that tethers could be used to link different parts of a space station together.

His work inspired the current tethered satellite project, which was originally scheduled to fly in 1988 but was delayed by the 1986 Challenger explosion.